1. Field of Invention
The present invention relates to power consumption of electronic equipment and maintaining linearity in amplifiers used in electronic circuits. The invention particularly relates to power consumption reduction and linearity requirements in mobile electronic devices such as CDMA communication devices.
2. Discussion of Background
There are presently multiple types of cellular radiotelephone systems operating. These systems include the advanced mobile phone system (AMPS) and the digital cellular systems including time division multiple access (TDMA) and code division multiple access (CDMA).
In cellular telephones and other communication devices, radio frequency (RF) amplifiers are typically used to amplify RF signals received by the devices. These RF amplifiers typically generate a gain of the received signal. One requirement is linear amplification is desired to prevent signal distortion. And, especially for mobile devices such as cellular telephones, due to the limited quantity of energy stored in the accompanying battery, efficiency is also a strong consideration.
However, efficiency and linearity are often competing considerations. One method for increasing efficiency is selecting an appropriate bias current for the transistor of the RF amplifier. When a low amplifier transistor bias current is chosen, battery life and talk time are increased. This generally results in acceptable distortion at low to moderate power levels, but creates unacceptable distortion at high power levels. When linearity is more important, a larger transistor bias current is chosen, reducing distortion to an acceptable level even at high power levels. The high bias current may also be required to obtain the maximum output power from the amplifier output transistor. However, the high bias current reduces battery life and talk time, particularly at low power levels.
Cellular radiotelephone systems operate by having multiple antennas covering a geographic area. The antennas radiate into an area referred to in the art as a cell. The AMPS cells are separate and distinct from the CDMA cells. This makes it likely that the antenna for one system's cell may be located in a cell of another system. Likewise, within a particular system (AMPS, CDMA, and TDMA), there are two service providers within a given area. These providers often choose to place cells in different geographical locations from their competitor, hence there are situations where a radiotelephone on system ‘A’ might be far away from the nearest system ‘A’ cell antenna while close to a system ‘B’ cell antenna. This situation means that the desired receive signal will be weak in the presence of strong multi-tone interference.
This intermixing of system antennas can cause problems for a mobile radiotelephone that is registered in one system, such as the CDMA system, and travels near another system's antenna, such as an AMPS antenna. In this case, the signals from the AMPS antenna can interfere with the CDMA signals being received by the radiotelephone due to the proximity of the radiotelephone with the AMPS cell or the higher power of the AMPS forward link signal.
The multi-tone interference encountered by the radiotelephone from the AMPS signals or other digital system creates distortion products or spurs. If these spurs fall in the band used by the radiotelephone, they can degrade receiver and demodulator performance. One way in which degradation occurs is the additional stress placed on RF amplifiers and mixers used in the RF chain that processes received signals.
RF chain electronics are similarly stressed when nearby systems are unintentionally ‘jammed’ the competitor system unintentionally. The goal of the cellular carrier is to provide a high signal to noise ratio for all the users of their system by placing cells close to the ground, or near their users, and to optimize coverage with the minimum number of cells resulting is so called “strong cells.” Unfortunately, this technique provides for better signal quality for the carrier's system at the expense of interfering with the competitor's system. The amplification of ‘jamming’ signals causes additional power drain.